1. Field of the Invention
The present invention relates to a method of casting a semi-liquid or semi-solid iron-based alloy, and a die for casting, which is used for such casting.
Priority is claimed on Japanese Patent Application No. 2008-155991, filed on Jun. 13, 2008, the content of which is incorporated herein by reference.
2. Description of Related Art
There is a die-casting technique as a technique for producing large amounts of metal parts having complicated shapes. This technique is to force molten metal under high pressure into a die to solidify the molten metal, and is effective as a method of producing metal parts having a low melting point, such as aluminum-based alloys and magnesium-based alloys. However, since iron-based alloys have a high melting point and the same iron-based alloy is frequently used as a die material, the die-casting technique has not been widely used to produce iron-based alloy parts.
In recent years, a method has been developed and put into practice which pays attention to the high strength of steel to produce metal parts from semi-liquid cast iron by using the die-casting technique. In addition to this, the development of dies having sufficient durability has been desired. Meanwhile, conventionally, various techniques have been developed to improve the durability of the dies, which are used in die-casting or injection forming, focusing on the use of nonferrous metals.
In particular, Japanese Unexamined Patent Application, First Publication No. 2004-114151 discloses a technique of performing a gas sulphonitriding treatment on the surface of a die to reduce the wettability of the surface of the die with respect to molten magnesium which is provided for die-casting or injection forming to thereby prevent seizure and improve the releasability of a cast product.
In addition, Japanese Unexamined Patent Application, First Publication No. 2001-232443 discloses a method of applying and drying a coating agent including particles such as fluorides, borides, carbides and carbonates, fats and oils, and organic metals on the surface of a die-casting die to form a coating having pores.
Japanese Unexamined Patent Application, First Publication No. 07-303933 discloses a technique of applying and drying a covering material, in which a powder such as an oxide and a fibrous material such as potassium titanate are dispersed in the water including sodium silicate, on the surface of a die for casting and performing a heating treatment for curing to conduct a covering treatment to thereby improve the durability of the die and the releasability of a cast product.
The temperature of semi-liquid cast iron is lower than the melting point of the cast iron. For example, the temperature of semi-liquid cast iron, in which the C content is 2.0%, is in the range of about 1200 to 1270° C., and is significantly higher than the melting point of an aluminum alloy or a magnesium alloy. In addition, the material which is frequently used in a die is die steel which is typified by SKD61. From these circumstances, a die is exposed to an environment, in which wear, seizure between the die and the cast product, cracks caused by thermal shock at the time of contact of the cast product, dissolution into the cast iron, and the like are significantly likely to occur. In fact, normal die-casting dies for an alloy of a low melting point withstand the production of more than ten thousand cast products. However, even when employing the techniques of Japanese Unexamined Patent Application, First Publication No. 2004-114151, No. 2001-232443, and No. 07-303933, current die-casting dies for semi-liquid cast iron have a durability limit corresponding to the production of about a thousand products. The die-casting dies for a semi-liquid cast iron have a short lifespan and an improvement in their durability is thus desired.
The present invention is contrived in view of these problems, and an object of the present invention is to provide a method of casting a semi-liquid or semi-solid iron-based alloy, in which wear at high temperatures in the inner surface of a die and seizure and corrosion caused by cast metal are prevented from occurring, and the durability is improved with good releasability in thixocasting (semi-liquid casting) and rheocasting (semi-solid casting) of an iron-based alloy (such as hypoeutectic cast iron), and a die for casting.